* remember to give an explanation
* XOR vs OR

  a b   OR
 -----|----
  0 0 | 0
  0 1 | 1
  1 0 | 1 
  1 1 | 1

 
  a b   XOR
 -----|----
  0 0 | 0
  0 1 | 1
  1 0 | 1 
  1 1 | 0 

xor rax, rax    ; A <- 0

A - B = A + (-B)

To get -B, use 2's complement
B = 10101011

1's comp  of B  01010100
2's comp  of B +       1
               ---------
                       
  0000000 
  01010100
 +       1
 ---------
  01010101  = -B


Suppose that myBytes is stored at 0000. 
How do you tell the computer that you want 12 and not 0000?

mov ax, myBytes     ; put the address (0000) into ax
max ax, [myBytes]   ; put the value (12) into ax

address  memory value
   0000  12
   0001  03
   0002  34 
   0003  45
   0004  5F
   0005  67
   0006  78
   ...
   FFFE
   FFFF